1. Field of the Invention
The present invention relates to production and use of materials that can be used with turbine airfoils. In particular, the present invention is directed to low density nickel-base superalloys with improved specific creep resistance and strength properties.
2. Description of Related Art
Nickel-base superalloys are used in the construction of some of the components of gas turbine engines that are exposed to severe temperatures and environmental conditions in the engines. For example, the turbine blades and vanes, seals, and shrouds are typically formed of such nickel-base superalloys. During service, these components are exposed to temperatures of 2000° F. or more. To perform at this high temperature for many engine cycles, the materials used in the components must have good rupture strength, a sufficiently high melting point, good thermal shock resistance, and good oxidation resistance at such high temperatures.
Turbine blades have been made from nickel-base superalloy single crystals for over twenty years. The first generation of single crystal superalloys contained no rhenium (Re). Second generation alloys typically contained 3 w/o Re and have attained successful application in commercial and military aircraft engines. Examples of these alloys include Rene N5, CMSX-4, and PWA 1484.
Third generation alloys were designed to increase the temperature capability and creep resistance further by raising the refractory metal content and lowering the chromium (Cr) level. These alloys had Re levels of ˜5.5 weight percent (w/o) and Cr levels in the 2-4 w/o range. Examples of these alloys include Rene N6 and CMSX-10. A fourth generation alloy (EPM 102) was developed in the 1990's with NASA sponsorship; it is a very strong alloy due to the increased levels of rhenium and other refractory metals. EPM 102 is considered to be the state-of-the-art.
Second generation alloys are not exceptionally strong, although they have stable microstructures and good oxidation resistance. Oxidation resistance has been achieved in second generation alloys with either yttrium additions or low sulfur (<1 ppmw or 0.0001 w/o) contents. Low sulfur contents can be commercially produced in castings by using melt desulfurization or by using effective hydrogen annealing after the casting has been directionally solidified.
Third and fourth generation alloys have considerably stronger creep resistance due to the use of high levels of refractory metals in the alloy. In particular, high levels of tungsten, rhenium, and sometimes ruthenium are used for strengthening in these alloys, and these refractory metals have densities much higher than that of the nickel base.
The impact of these refractory additions is that the overall alloy density is significantly increased, such that the fourth generation alloy is about 6% heavier than second generation alloys. This weight increase may seem small, but any weight increase to the blade also cascades to the disk, shaft, etc., and increases the overall vehicle system weight by a factor of 8 to 10×. High alloy densities limit the use of the superalloy, and third and fourth generation alloys are used only in specialized applications.
The use of third and fourth generation alloys is also limited by microstructural instabilities which can debit long-term mechanical properties. The alloys are sometimes more difficult to manufacture due to additional processing steps that are needed to mitigate these microstructural instabilities. Additional processing steps add cost to the manufacturing of these alloys, and unfortunately these steps are not always successful in eliminating these instabilities. Thus, there is a need in the prior art for a unique alloying approach in order to achieve microstructural stability, high creep resistance and strength in a turbine blade alloy with low density.